One solution to the hot corrosion and oxidation problem which is widely applied in gas turbine engines, is to alloy aluminium into the surface of a superalloy component, a process known as aluminizing. Aluminium forms stable intermetallic compounds with both nickel and cobalt. The oxide layer which forms on these compounds at high temperature is no longer a metal oxide of nickel or cobalt, but rather a tough, tightly adherent, protective layer of alumina, Al.sub.2 O.sub.3 (FIG. 3).
A variety of commercial coatings are based upon this protection scheme. Sometimes aluminium is deposited from a vapour phase in a process that has come to be known as pack aluminizing. In pack aluminizing, aluminium powder is reacted with halide activators to form gaseous compounds which condense on the metal surface and react producing aluminium metal. The aluminium atoms diffuse into the substrate, reacting to produce intermetallic aluminides. This process has been described in detail in a number of patents, including U.S. Pat. No. 3,257,230 (Wochtell et al).
State-of-the-art MCrAlY overlay coatings also rely upon alumina films for their hot corrosion resistance. Owing to the presence of chromium and yttrium in the film, aluminium contents in these coatings do not need to be as high as in pack aluminides; however, protection is still derived from a tightly adherent scale of alumina.
Slurry aluminizing is another alternative method of providing a protective, alumina forming intermetallic aluminide coating on a superalloy. In the slurry process, an aluminium- filled slurry coating is first deposited on the hardware. When the coated part is heated in a protective atmosphere, aluminium in the film melts and reacts with the substrate to form the desired intermetallic phases.
The demonstrable resistance of aluminide coatings to hot corrosion and oxidation is due to the thermodynamic stability of the alumina scale that forms on them. However, they do have some susceptibility to "low temperature" hot corrosion attack at about 700.degree.-800.degree. C. by alkali metal oxides (e.g. Na.sub.2 O) and acidic oxides of refractory metals (e.g. MoO.sub.3 and W.sub.2 O.sub.3).
Silicon dioxide (SiO.sub.2) is another very stable oxide. Like aluminium, silicon forms stable intermetallic compounds (silicides) with nickel and cobalt as well as chromium and other elements typically found in refractory alloys, such as molybdenum, tungsten and titanium. This reduces the segregation of these elements into the outer surface protective oxide layer, thus improving its protectiveness. Furthermore, unlike aluminium, silicon is unable to form sulphides and is resistant to sulphur diffusion. Consequently, silicide coatings, produced by pack or slurry processes, have been used on refractory alloys to improve resistance to hot corrosion and oxidation. Silicides have proven particularly useful in resisting sulphurous attack at "low" temperatures (700.degree.-800.degree. C.). The benefits of silicon-based coatings have been described by many, including F. Fitzer and J. Schlicting in their paper "Coatings Containing Chromium, Aluminium and Silicon for High Temperature Alloys", given at a meeting of the National Association of Corrosion Engineers held Mar. 2-6, 1981 in San Diego, Calif., and published by them as pages 604-614 of "High Temperature Corrosion", (Ed. Robert A. Rapp). This paper is included herein by reference.
The benefits of aluminizing and siliconizing are combined in processes which simultaneously deposit both aluminium and silicon on a metal surface, usually that of a superalloy. One such process, described in U.S. Pat. No. 4,310,574 (Deadmore et al), deposits a silicon-filled organic slurry on a surface, then aluminizes the surface by a conventional pack aluminizing. Aluminium carries silicon from the slurry with it as it diffuses into the superalloy from the pack mixture. Deadmore et al ('574) demonstrates that the resultant silicon-enriched aluminide has better resistance to oxidation at 1093.degree. C. than did aluminides without silicon.
Another means to produce so-called "silicon-modified" or "silicon-enriched" aluminides is to apply a slurry containing elemental aluminium and silicon metal powders to an alloy substrate containing aluminide and silicide forming elements and then heat it above 760.degree. C. (1500.degree. F.). As the aluminium and silicon in the slurry melt, they react with the substrate elements and diffuse preferentially. The aluminium alloys with nickel or cobalt in the substrate while silicon alloys with chromium or other silicide formers. The end result is a composite aluminide-silicide coating. This process is often termed a silicon modified slurry aluminide process and is commercially utilised under the trade name, "SermaLoy J".
In test, this silicon-modified aluminide coating proved uniquely resistant to sulphidation attack over a wide range of operational temperatures. Details of some testing has been published by American Society of Mechanical Engineers (ASME) in a paper by F N Davis and C E Grinell entitled "Engine Experience of Turbine Materials and Coatings (1982) which is incorporated herein by reference. This coating is now specified on many industrial and marine turbines.
Experience suggests the silicide phases are key to the enhanced corrosion resistance of this aluminide-silicide coating, because they displace some of the vulnerable aluminide phases from the surface layer. Unfortunately, particularly when utilised on superalloys, these critical silicide phases become excessively concentrated in the outer third of the coating microstructure after a typical coating and diffusion treatment. Silicon content of the outer surface can be as high as about 14-17 wt. %, as opposed to 8 wt. % in the bulk of the coating. This seems to render the outer part of the coating prone to cracking after long service. Crack propagation is rapid after crack initiation, even though the threshold for initiation is high. Although the cracks are not very serious, in that they do not propagate into the superalloy substrate, it would be preferable to prevent their occurrence or restrict their penetration through the coating, since they eventually open up corrosion paths to the substrate.